1. Field of the Invention
The present invention relates to a drive circuit for driving an optical modulator and more particularly to a drive circuit for driving a DQPSK optical modulator.
2. Description of the Related Art
Phase modulation is widely put in practical use as one technology for transmitting signals in an optical transmission system. In phase modulation, data is transmitted by controlling the phase of a carrier wave according to transmission data. For example, in quadrature phase shift keying (QPSK) modulation “θ”, “θ+π/2” “θ+π” and “θ+3π/2” are assigned to symbols “00”, “10”, “11” and “01” each composed of two-bit data, respectively. In this case, “θ” is an arbitrary phase. The receiver recovers the transmission data by detecting the phase of the reception signal.
Differential QPSK (DQPSK) is known as a technology for fairly easily realizing a QPSK receiver. In DQPSK modulation, corresponding phases (0, π/2, π and 3π/2) are assigned to differentials between two continuous symbols. Therefore, the receiver can recover transmission data by detecting the phase difference between two continuous symbols.
FIG. 1 shows the configuration of a general DQPSK transmitter. Here only components needed to explain its basic operation are shown.
A DQPSK signal modulator 10 is a Mach-Zehnder modulator and continuous wave (CW) light generated by an optical source 1 is applied to it. The CW light is split by an optical splitter and guided to a pair of arms 11 and 12. The arm (I arm) 11 is provided with a phase modulator 13 and the arm (Q arm) 12 is provided with a phase modulator 14 and a phase shifter 15. The phase modulators 13 and 14 are Mach-Zehnder modulators. The phase shifter 15 gives phase difference π/2(π/2+2nπ (n is an integer)) between the arms 11 and 12.
A drive signal generation unit 2 generates a pair of drive signals Data 1 and X_Data 1, and a pair of drive signals Data 2 and X_Data 2 by encoding the transmission data using a DQPSK precoder. The drive signals X_Data 1 and X_Data 2 are the inverted signals of the Data 1 and Data 2, respectively. The drive signals Data 1 and X_Data 1 are applied to the phase modulator 13 as differential signals after being amplified by driver elements 21a and 21b. Similarly, the drive signals Data 2 and X_Data 2 are applied to the phase modulator 14 as differential signals after being amplified by driver elements 22a and 22b. Specifically, the phase modulators 13 and 14 are driven by a pair of the drive signals Data 1 and X_Data 1 and a pair of the drive signals Data 2 and X_Data 2, respectively. Then, by combining a pair of optical signals outputted from the phase modulator 13 and 14, DQPSK signals are generated.
FIG. 2 explains the operation of the DQPSK transmitter. In this case it is assumed that the phase of CW light is zero.
The phase of an optical signal outputted from the phase modulator 13 is controlled to “0” or “π” according to the drive signals Data 1 and X_Data 1. The arm 12 is provided with the phase shifter 15 with the amount of phase shift π/2. Therefore, the phase of an optical signal outputted from the phase modulator 14 is controlled to “π/2” or “3π/2” according to the drive signals Data 2 and X_Data 2. Thus, “π/4”, “3π/4”, “5π/4” or “7π/4” is assigned to each symbol (00, 10, 11 or 01) having two-bit information. Then, the receiver recovers the transmission data by detecting a phase difference between two continuous symbols.
For example, Patent Document 1 (Japanese Patent Publication No. 2004-516743) discloses in detail the configuration and operation of a DQPSK optical transmitter.
When the intensity of the optical signals on I and Q arms are imbalanced in the DQPSK transmitter with the above-described configuration, as shown in FIG. 3, the phase of a DQPSK optical signal obtained by combining them becomes inappropriate. Then, a possibility of wrongly detecting the phase of the optical signal in the receiver becomes high and channel quality deteriorates. One factor of this problem is the manufacturing variability among modulators (modulators 13 and 14 in FIG. 1)
In order to improve channel quality the above-described problem must be solved. However, DQPSK is still a technology under development and there are few specific proposals.